5 B Boron

Click image to magnify. Pure crystalline boron, front and back side. Original size in cm: 2 x 3
Boron is not a very frequent element and is found in nature only in compounds together with oxygen. Crystalline boron, which is shown here, is nearly as hard as diamond (9.5 on Mohs scale, diamond has 10). Boron has different biological functions. Boron compounds have many, often very special applications, a common one is B2O3 for borosilicate glass. Most famous probably are the perborates as bleach and as washing agent. Because of their bad biodegradability, the use of perborates has gone back.

Left: Amorphous brown boron, another boron allotrope, as powder in a glass jar. 5 grams, jar diameter 4 cm.
Right: Ulexite, a complex boron mineral and one of the most important ores for boron mining. This one is from the borax mine near the town Boron in California. The orange inclusions are realgar.